NEW YORK—William Bratton on Thursday was named the next commissioner of the New York Police Department, giving him a second turn at the helm of the nation's largest municipal law-enforcement agency.

Mr. Bratton's career in law enforcement, spanning nearly 46 years, already includes leading the police departments in Los Angeles, Boston and New York—a distinction no other U.S. police chief can claim. He oversaw significant reductions in homicides and other violent crimes during his first term as commissioner, from 1994...